The present invention relates to a process for the removal of mercury from sulfur dioxide-bearing hot and moist gases, especially roasting-plant or smelting-plant gases emerging from a hot electric filter, by cooling and scrubbing the hot gases by means of a strong sulfuric acid solution in order to sulfatize the mercury and to separate it from the gases.
Sulfidic concentrates of zinc and copper normally contain mercury from a few parts per million to a few hundred parts per million. In the roasting-plant and smelting-plant treatments of these concentrates, the mercury compounds decompose and the mercury is present in the gas in the form of metal vapor. In this case, the mercury sulfide of the concentrates reacts as follows: EQU HgS(s)+O.sub.2 (g).fwdarw.Hg(g)+SO.sub.2 (g). (1)
In the case of concentrates the mercury concentration of which is between 10 and 1000 g/t, the concentration of mercury in the corresponding gas phase is in the order of 5-500 mg/m.sup.3 (0.degree. C., 1 bar). After emerging from the roaster or smelting reactor, the gases are normally cooled in waste heat boilers and the dusts are removed in cyclones and electric filters.
During these stages, the temperature of the gas is in the order of 300.degree.-350.degree. C., at which the mercury is still in the gas phase. The SO.sub.2 -bearing gases produced in the roasting or smelting of sulfidic ores are generally used for the production of sulfuric acid. After the treatment stages described above, the gases are in this case directed through a scrubbing tower, an indirect cooling unit, a wet electric filter, a drying tower and a heat exchanger to a contact unit and an absorption tower. In the scrubbing tower the gases are scrubbed by means of 10-30% sulfuric acid, at which time the temperature of the gas drops to 40.degree.-60.degree. C. After indirect cooling and the filtration through a wet electric filter, the temperature of the gas is about 30.degree. C. During these stages, part of the mercury passes out of the gas and remains in the scrubbing precipitates of the scrubbing towers. The remainder--usually most--of the mercury passes, in elemental form, together with the gas to a drying and absorption tower, in which it dissolves and then passes on into the produced acid. If it is assumed that the sulfidic concentrate contains mercury 100 g/t and that half of this amount is removed during the scrubbing stage, the produced acid contains mercury approx. 90 mg/l, i.e. approx. 50 g/t. Using the same calculation method it can be seen that a mercury concentration as low as 10 g/t in the concentrate is capable of increasing the concentration of mercury in the sulfuric acid to the order of 5 g/t. The uses of mercury-bearing sulfuric acid are limited. Standards have been set for the concentration of mercury in sulfuric acid, depending on the purpose for which the acid is used. Although maximum limits, prescribed by law, for the concentration of mercury in sulfuric acid have not yet been generally achieved, values of 1 g/t and 0.5 g/t have usually been proposed as such a limit; opinions have also been expressed that the limit should be lowered to a value as low as 0.2 g/t.
It is evident that such low concentrations of mercury in sulfuric acid will not be obtained for sulfuric acid produced from the SO.sub.2 gases of zinc-roasting plants and copper-smelting plants, unless the gas, or alternatively the produced sulfuric acid, is purified with respect to mercury.
Previously known is a mercury removal process in which the gas emerging from a hot electric filter is scrubbed by means of a 85-93% sulfuric acid solution. The temperature of the cycled sulfuric acid solution, saturated with metal sulfates and other metal salts, is adjusted during the scrubbing stage by means of the circulation rate of the solution and by means of external heat exchange, usually to 160.degree.-220.degree. C. In this case, practically all of the mercury present in the inlet gas is sulfatized, and the concentration and temperature of the sulfuric acid solution can be adjusted, within the above-mentioned ranges of concentration and temperature, to such levels that the partial pressure of water vapor in the sulfuric acid solution during the scrubbing stage is the same as the partial pressure of water vapor in the gas which is being scrubbed, whereby the concentration of the cycled sulfuric acid solution remains constant.
When the cycled sulfuric acid solution is saturated as regards mercury salts and other--typical of each system--metallic salts (metals present in the dust), these salts crystallize out continuously from the cycled sulfuric acid solution. The total solubility in sulfuric acid of mercury and the metals present in the dust is in the order of a few grams per liter. The solid material which is salted out is separated from the cycled sulfuric acid solution. The separated precipitate is washed, whereby the sulfuric acid and most of the metal salts present in it dissolve and the mercury remains in the final precipitate. The final precipitate usually consists of the compound Hg.sub.3 Se.sub.2 SO.sub.4 ; the scrubbed gases normally contain selenium in the form of selenium oxide, which in a cycled, SO.sub.2 -gas saturated and mercury sulfate-bearing sulfuric acid reaches the selenite degree and forms the above-mentioned compound with mercury and sulfate. If the amount of selenium present is small, HgSO.sub.4 is also produced. The mercury and the selenium are separated from the precipitate and recovered.
The process for the removal of mercury discussed above is described in Finnish Pat. No. 45 767, and the recovery of mercury and selenium is described in Finnish Pat. No. 48 708. The process is also described in the article by A. Kuivala, J. Poijarvi, "Sulphuric acid washing removes mercury from roaster gases" E/MJ --October 1978, 81-84. When the cycled sulfuric acid solution is saturated as regards mercury salts and other--typical of each system--metallic salts (metals present in the dust), these salts crystallize out continuously from the cycled sulfuric acid solution. The total solubility in sulfuric acid of mercury and the metals present in the dust is in the order of a few grams per liter. The solid material which is salted out is separated from the cycled sulfuric acid solution. The separated precipitate is washed, whereby the sulfuric acid and most of the metal salts present in it dissolve and the mercury remains in the final precipitate. The final precipitate usually consists of the compound Hg.sub.3 Se.sub.2 SO.sub.4 ; the scrubbed gases normally contain selenium in the form of selenium oxide, which in a cycled, SO.sub.2 -gas saturated and mercury sulfate-bearing sulfuric acid reaches the selenite degree
The gas emerging from the dry electric filter, the temperature of the gas being approx. 350.degree. C., is washed by means of a sulfuric acid solution cycled in a sulfatizing tower and cooled in a heat exchanger. The temperature of the gas drops to approx. 180.degree. C. The mercury present in the gas is sulfatized, the sulfur trioxide and the selenium dioxide remain in the cycled acid, the water vapor, the hydrogen chloride and the hydrogen fluoride pass through the tower. The SO.sub.2 gas emerging from the sulfatizing tower contains also a considerable amount of sulfuric acid, in addition to water vapor, hydrogen chloride and hydrogen floride. The gas is now scrubbed using an approximately 50% sulfuric acid solution, which is cooled in a heat exchanger. The temperature of gas drops to approx. 50.degree. C. The water vapor, the hydrogen chloride and the hydrogen fluoride pass through the tower, but most of the sulfuric acid remains in the tower. Finally, the gases are further scrubbed using a 5-10% sulfuric acid solution. The temperature of the gas drops at this stage to 30.degree. C. The hydrogen chloride, the hydrogen fluoride, the remainder of the sulfuric acid and part of the water remain in the tower. The gas continues its journey from the electric filter to the drying tower and from there on to the contact unit and the absorption tower.
By this known method, mercury can be removed from gas very effectively, but the price of this comprises the capital outlay for the sulfatizing tower and its accessories, as well as the expenses of the operation of the process. When the question has been of removing mercury from the gases emerging from zinc-roasting plants and copper-smelting plants, companies have usually not been willing to make considerable investments. This is the most important reason why the use of the mercury removal processes has not become widespread in these connections.
The central unit of the above-mentioned mercury removal process, known per se, is the sulfatizing tower and its accessories. In the further development of the process, special attention must be paid to ways in which this unit can be simplified and in which capital outlays for it can be minimized.